1. Field of the Invention
The present invention relates to an alignment device for fabricating an optical disc, in which the center of a thin film-like cover sheet is aligned with the center of a disc substrate when an optical disc is fabricated by sticking the cover sheet onto a recording surface of the disc substrate.
2. Description of the Related Art
Already widespread are optical discs such as CD-R (Compact Disc-Recordable), CD (Compact Disc), DVD (Digital Versatile Disc), and DVD-R (Digital Versatile Disc-Recordable) for recording or reproducing information by the use of a laser beam. In recent years, storage of increasing quantities of information, such as video information, has been demanded for optical discs, and therefore, high density of recording information is progressively being studied. Such information recording densities of optical discs generally depend upon a spot size of the light beam on the disc. The spot size is proportional to λ/NA, where λ designates a wavelength of the laser and NA denotes the numerical aperture of an objective lens. Consequently, in order to enhance the recording density of the optical disc, to be effective it is required that the wavelength of the laser beam be shortened, and further, that the NA of the objective lens be increased. Since a comatic aberration generated by inclination of the optical disc becomes larger in proportion to the cube of the NA, the margin with respect to inclination such as tilt of the disc becomes markedly small with higher NA. Therefore, the beam spot is blurred even with slight inclination, thereby making it impossible to achieve recording and reproduction at a high density. Hence, it is necessary to form a cover layer serving as a laser beam transmitting layer of a satisfactory thinness (for example, as thin as about 0.1 mm) in the conventional optical disc suitable for high density, and, it is necessary to suppress an increase in spatial aberration (comatic aberration) caused by inclination of the disc in association with high NA (for example, see Japanese Patent Application Laid-Open (JP-A) No. 11-31337, pages 9 and 10 and FIG. 4).
In a fabrication line of the above-described optical disc, for example, a thin film-like cover sheet made of a resin film is stuck onto a recording surface of a disc substrate, having an information recording layer previously formed thereon. Thus, a transparent cover layer is formed on the disc substrate by the cover sheet, thereby fabricating the optical disc. Here, the cover sheet is formed of a resin film and an adhesive film formed at one surface of the resin film. When the cover sheet is stuck onto the disc substrate, a laminate consisting of the cover sheet and the disc substrate (hereinafter referred to as “a disc laminate”) is first formed by laminating the disc substrate onto the adhesive film of the cover sheet. In this state, the entire adhesive film is not uniformly brought into contact with the recording surface of the disc substrate, and therefore, the cover sheet is not joined to the disc substrate by satisfactory joining force. As a consequence, in the fabrication line of the optical disc, the disc laminate is pressed along its thickness direction under a predetermined pressing force, so that the entire cover sheet is uniformly brought into contact with the recording surface of the disc substrate. Thus, the cover sheet in the disc laminate is stuck to the disc substrate by satisfactory joining force, thereby fabricating the optical disc to be used as a product material.
In an optical disc, such as fabricated as described above, it is necessary to align the center of the disc substrate and the center of the cover sheet with each other with satisfactory high degree of accuracy. Specifically, the eccentricity tolerance of the cover sheet with respect to the center of the disc substrate needs be set to about 100 μm.
One example of an optical disc fabricating apparatus for fabricating the above-described optical disc (hereinafter referred to as “an optical disc fabricating apparatus”) is disclosed in JP-A No. 2003-217192, page 8 and FIGS. 1 and 2. In the optical disc fabricating apparatus disclosed in JP-A No. 2003-217192, a cover sheet is vacuum-adsorbed to a biaxial alignment stage 2, to be then moved to an image processing position, at which CCD cameras and the like are disposed. Thereafter, at the image processing position, images at outer peripheral portions of the information recording region of the cover sheet are taken by the three CCD cameras arranged at intervals of 90°. Subsequently, in the optical disc fabricating apparatus, eccentric alignment is carried out by: moving the biaxial alignment stage in two axes (i.e., in the XY-axes directions) by comparison of the image information taken from the outer peripheral portion of the information recording region of the cover sheet with reference data; a disc substrate is supplied onto the biaxial alignment stage; a three-point clamp device is inserted into a center hole formed in the disc substrate; and the disc substrate is centered (that is, center alignment) while the disc substrate is clamped by the three-point clamp device. After that, in the optical disc fabricating apparatus, the cover sheet and the disc substrate are housed inside a vacuum tank together with the biaxial alignment stage, and then, the pressure inside of the vacuum tank is reduced to a predetermined degree of vacuum. Thereafter, the disc substrate is pressed toward the cover sheet on a support table by a pad serving as a pressing member having elasticity in the vacuum tank, so that the cover sheet and the disc substrate are stuck to each other.
However, in order to align the center of the cover sheet and the center of the disc substrate with each other with satisfactory accuracy, the above-described optical disc fabricating apparatus needs be provided with: a plurality of CCD cameras; an information processor for processing image information obtained by the CCD cameras, so as to produce positioning control information; and an alignment stage which can be positionally adjusted in two axes with high accuracy in accordance with the control information output from the information processor. Therefore, problems arise since the above-described optical disc fabricating apparatus includes a complicated mechanism (i.e., an alignment mechanism) for aligning the center of the cover sheet and the center of the disc substrate with each other with the satisfactory accuracy. Furthermore the time required, for aligning the center of the cover sheet and the center of the disc substrate with each other with satisfactory accuracy, becomes long.